The invention relates to a method and circuit arrangement for controlling the motor current in an electric motor, in particular a stepper motor, by means of a chopper method.
It is known, using a chopper method, to generate from a supplied motor supply voltage the direction of current, level of current and shape of current (in microstep operation usually a sine shape or cosine shape) for each motor coil of a stepper motor to be memorized in each coil according to a corresponding specified current (target coil current) using PWM pulses in order to drive the rotor of the motor.
In doing so, three different coil current phases are differentiated that are activated by means of the chopper method:
During the ON phase, the coil current in a coil is actively driven through the coil in the momentarily specified polarity or, respectively, direction of the coil current so that the coil current amount increases relatively quickly and continuously (startup period). This means the coil current direction that is memorized by way of an ON phase corresponds to the momentary polarity or, respectively, direction of the coil current.
In case of a sine-shaped coil current the polarity of the coil current is positive in the first and second quadrants and negative in the third and fourth quadrants, for example.
In the fast decay (FD) phase, the coil current is reduced against the respective specified polarity of the coil current by reversing the polarity of the coil and feeding the coil current back into the current supply. The FD phase is used to reduce the coil current relatively quickly, especially in the phase of a decreasing coil current amount (i.e. during the second and fourth quadrant of a sine-shaped coil current) and to prevent an adulteration of the specified current, in particular due to the CEMF.
The third phase of the chopper operation is the recirculation phase or slow decay (SD) phase, in which the doll is not controlled actively but rather is short circuited or bridged so that the coil current only decreases gradually (i.e. slower than during the FD phases) due to the internal resistance of the coil and the CEMF.
This means the chopper activates, measures and combines these three chopper phases with regard to time by means of chopper switching signals (usually PWM signals) supplied by a motor driver circuit so that the actual coil current follows a specified current (target coil current) as contemporaneously and exactly as possible across its entire (e.g. sine-shaped) course, i.e. during the increasing and decreasing current phases, and in particular is not changed (much) by the counter induced voltage (counter CEMF) caused by the rotor in the motor coils. This requires that the actual coil current be suitably measured or determined.
Due to the inaccuracies affiliated with such measuring or determination processes as well as due to the customary scattering of the electric component properties of the motor driver circuits as well as the internal resistance and the inductivity of the motor coils, the actual course of the coil current is always at least somewhat different than the specified target course of the coil current. Other reasons for this furthermore is the more or less inaccurate capture of the coil current measuring values, which is due to practical compromises, such capture only being possible during the ON and FD phases for bridge foot point measuring, for example. In particular, however, the chopper principle,that is used, i.e. the way in which the ON, FD and SD phases are controlled based on the measuring values, always result in a certain deviation of the actual course Of the coil current from the target course of the coil current. This is due to the discrete working frequency, which is limited due to practical and physical reasons, and thus also is due to the chopper frequency. It was found that such deviations, in particular in the area of the zero crossing of the actual coil current, can cause disadvantageous effects, for example by the motor making a louder running noise and displaying resonances and poorer positioning properties.
It is desirable to provide a method and circuit arrangement for controlling the motor current in an electric motor, in particular a stepper motor, by means of a chopper method, by means of which Method/circuit arrangement the actual course of a coil current, in particular in the area of its zero crossing, can be adapted to a specified target coil current course substantially more accurately and least to the extent that the above referenced disadvantageous effects are no longer perceptible or can be negligible for a specific application.
A method for controlling the motor current in an electric motor according to an aspect of the present invention is provided, in particular a stepper motor, by means of a chopper method, by means of which method a coil current is controlled by means of at least one of the motor coils (A,B) during a chopper phase in the direction of a first target current value that is larger or smaller by a specified amount than a momentary amount of a specified target coil current, whereby the chopper phase is completed when the value of the coil current reaches the first target current value and whereby the first target current value is approximated to the momentary amount of the specified target coil current during the chopper phase in a manner that ensures that the chopper phase is completed prior to the expiration of a predetermined maximum duration.
A circuit arrangement for controlling the motor current in an electric motor according to an aspect of the present invention comprises a chopper (CH) for triggering a motor driver circuit (DR), a comparator (K) for comparing the target current values with actual current values, whereby an output of the comparator (K) is connected to the chopper (CH) as well as a device, for generating the target current values.
The principle according to the an aspect of invention can be applied to 2 phase stepper motors as well as to 3 and multi-phase stepper motors.
Additional details, characteristics and advantages of the invention are described in the following description of preferred, exemplary embodiments based on the drawing. The following is shown: